All stainless alloys display a decrease in ductility in sub zero temperatures.

However, that is basically why is so vitally important that whoever manufactures the part nail the heat treat. If the strength (measured for this purpose by hardness RC scale) is sufficient that strain never reaches the critical failure point, then the ductility of the steel is immaterial. It could be rediculously brittle, but if the hardness is up to withstanding the strain, it doesn;t matter.

Bear in mind that diamond is not only one of the hardest materials on earth, it is also one of the most brittle. Almost zero ductility at all. Ever try breaking diamond with a hammer? I have. It CAN be done, but boy oh boy is that rock HARD! Once you hit it hard enough to break it, it just comes apart, all at once. Brittle steel is like that. Depending on how hard it is, it may not matter one iota that it's brittle, but if it isn't hard enough, it'll break.

The few actual anecdotes I've ever heard of stainless actually failing in cold weather were all on factory barrels. I don't trust the QC on the heat treating of parts on such a massive scale.

A good custom though, is another story. While I still feel that much better product could be achieved by treating one at a time to a custom HT regime, heat treating shops typically do a much better job at QC than factory operations with HT facilities built in.

# What does "polygonal rifling" mean? How is it different from other
# kinds? Is it a process or a pattern or what? Thanks in advance.

Instead of the rifling being square and hanging down in the bore so that
it can engrave a square notch in the bullet it has flats where the
rifling would be. The bullet is not upset much. Instead of having 6 or
8 sharp knotches it will have small flats that are very unnoticeable.
This makes the bullet fly better in the wind because there is no sharp
edges to bite into a cross wind. The bullet jacket is not deformed as
much so the chances of loose cores are much less. Since there is no
sharp corners to burn off the barrel life is much longer and is
dependent on heat checking alone to end its life. That is one major
reason most barrel makers are not interested in them. It would decrease
their business by half. They are more difficult to make than regular
rifled barrels for a couple of reasons one being they are difficult to
lap and give a cut barrel maker more problems due to tooling.

The lining of a barrell with chromium CAN protect it from heat checking, but the abbrasion, pressure, etc. will eventually cause pitting in the chromium liner. What happens is this:

When the bore is lined with chromium, the process is VERY hard on the new chromium liner which is riddled with microscopic (as in electron microscope) cracks from the forming process.

For a while everything seems hunky dory, as the barrel is rediculously easy to clean, doesn't foul even half as much as a lapped barrel, and shoots with the best of them.

Then, somewhere around the same time as a normal barrel would start losing it's premium accuracy life, and thus be retired to fireforming, load development, and practice duty (for a benchrester)the cracks in the chromium have grown big enough that bits of the liner start peeling down the barrel embedded in the sides of bullets or carried by the expanding propellent. This causes two major flaws in chrome lined barrels. 1: the throat area which would normally erode more evenly seems to get VERY rough all of a sudden and 2: the bore gets scratched and pitted to hell by the chrome schrapnel being dragged down it's length at speed!

Chrome barrels are GREAT within their accuracy life, but when one gives up the ghost, it happens all at once. The accuracy life is really not any longer than with other barrels, as the lining process leaves an imperfect liner that is just waiting for those cracks to propagate far enough to let the real damage begin!

This is why I'm so interested in the idea of gas nitriding a bore. Chromium is partially used because it is so much freaking harder than the substrate steel (barrels are quite soft as steels go). Gas nitriding trates the steel surface to a thickness of several thousanths to be even much harder than chrome! The bore would have to be PERFECT before the nitriding, and you would want to repolish with VERY fine (lapidary scale) abrasives after, but so far, I cannot find a good, solid metallurgical explanation as to why this hasn't yet become standard practice.

PVD coatings are VERY promising as well, in fact, they are harder still, and the PVD process doesn't hurt the new lining like the chrome plating process does, but PVD deposition technology is still in its infancy, and hasn't yet been capable of coating the inside of a long narrow tube, the machinery just can't fit!

It appears, after further reasearch that not all poly barrels are created equal! Some are buttoned into shape, and others hammerforged around a "negative". The hammerforged type (used by some militaries, and extensively in handguns) have little to reccomend them in the way of accuracy potential, as the hammerforging process is NOT good for grain structure of steel. With a good normalization regime in the heat treat, it may be a viable option, but nobody seems to want to go to that much trouble...

The buttoned type, of good quality seems to be just as accurate as the rest of the top quality barrels we are all familiar with, and seem to offer rediculously extended life! Increase velocity and easier cleaning are "side benefits".

For high volume shooting, carbon / graphite barrels help dissipate heat MUCH faster. This, also will translate into at least a slight increase in barrel life, as heat is enemy # 1. Even in low volume shooting, if you burn a lot of powder, this should help shorten (by fractions of a millisecond) the amount of time the steel surface is hot enough to carburize.

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IMHO this is total crap and propaganda, from what I have seen through a bore scope if you have a high intensity cartridge (like every gun I own except 222Rem and 6mmBR) your throat firecracks before it starts to erode forward. a 3groove is much rougher on a bullet in a firecracked state than a 4,5,6 groove barrel ,so it in reality has a shorter barrel life.